I did it about 5 years ago and lost 2 stone. It may have changed a bit since then, but the gist of it is this:
You have a list of “free food” that you can eat as much as you want of. This includes fruit and veg, lean meat, pasta, rice, potatoes etc. Also fat free yogurts, cream cheese and stuff.
Then you have “healthy extras” which you allowed a limited amount of of. Cereals, bread, and dairy stuff. (I think there are 2 lists of healthy extras, and you are allowed one of each)
Then everything else has a “syn” value
and you have a daily syn allowance. So if you want cake, for eg, then you will use you syn allowance.
I used to save my syns up until the weekend and then use them all on wine 
Anyway, it worked for me, you get used to what is free and what’s not so is quite easy once you get the hang of it.
I did put all the weight back on and more after, but that was mainly down to having DS. I’m currently jut calorie counting using mfp and loosing at a similar rate to when I was on slimming world.